This invention relates to an improvement in fabrication of multifilament superconductors, and more particularly, to a method for fabricating multi-filament Nb.sub.3 Sn superconductors.
In the art of this field, for the fabrication of multifilament Nb.sub.3 Sn superconductors are generally well known two methods, one being called as a bronze technique in which a copper-tin (bronze) alloy including 13-14 weight % tin is utilized as a matrix material in which a niobium or niobium based alloy rod is arranged. Reducing operation is applied to the thus made composite under the repeated heating and annealing operations, and thereafter, the composite material is heat treated to thereby form Nb.sub.3 Sn layer in the composite. However, difficulties associated with this bronze technique lie in that because the high extent of work hardening is required for the bronze alloy including 13-14 weight % of tin, one intermediate annealing under a temperature of about 500.degree.-650.degree. C. would be required every time when the reducing working rate reaches 40-60% during the reducing operation in order to lower the hardness of the bronze alloy increased during the working, which requires a large number, i.e. several tens of times, of the intermediate annealings throughout the entire working operation, and in addition, this treatment must be done in a non-oxidizing atmosphere, which requires specific equipments or treatment therefor, thus being disadvantageous.
In addition, it is known that the upper limit of Sn content in the bronze alloy is about 14 weight %, and when the Sn content increases over this upper limit, a breakage may occur in the working operation thereof, and in a certain case, it may be difficult to work the bronze alloy even if the intermediate annealings are carried out. In order to eliminate such intermediate annealing processes, several methods have been proposed in the art of this field such as disclosed in U.S. Pat. Nos. 4,435,228, 4,419,145 and 4,385,942 to Tachikawa et al.
One method is generally called an internal diffusion method in which Sn core is disposed at substantially the central portion of a composite including an Nb based alloy rod disposed in Cu matrix, and the thus prepared material is then subjected to reducing operation. Thereafter, a preheating procedure is carried out and diffusion heat treatment is then performed for forming an Nb.sub.3 Sn layer, thereby to obtain a multi-filament superconductor. In this method, however, it is required to carry out the preheat treatment for diffusing evenly throughout the Cu matrix, i.e. whole Nb filaments, this preheat treatment requiring much time such as about one month, thus being not effective and economical. Moreover, in the internal diffusion method, the Sn in the Cu-Sn composite might be dissolved during the pretreatment in a case where the Sn content is over about 20% in surface area ratio, which results in less workability.
The other method is generally called an external diffusion method in which a composite including the Nb based alloy rod arranged in the Cu matrix is reduced, and the Sn plating is then effected. The composite is preheated to thereby form the Nb.sub.3 Sn layer by means of diffusing heat treatment, thus obtaining composite superconductor. In this external diffusion method, however, the preheating treatment for diffusing the Sn entirely in the Cu matrix requires much time, and moreover, the diffusing distance is limitted to an extent of about 0.7 mm, so that it is considerably hard to manufacture a superconductor through which large current can pass. In addition, the technique for thickening the thickness of the Sn plating to increase the Sn content in the Cu-Sn composite includes a problem of excessive fusing of Sn during the preheating procedure. Accordingly, in the external diffusing method, the Sn content is limitted to about 14 weight % as in the bronze method described hereinbefore.
There is further proposed a method generally called an Nb tube technique, such as disclosed in German DT No. 2,620,271, in which a material requiring no work hardening for eliminating intermediate annealings is utilized and in which a tin rod clad with copper is located inside the niobium tube and a copper is also placed on the outer surface of the niobium tube as a stabilizer. A plurality of composites thus made are bundled and subjected to the reduction operation and then heat treated to form the Nb.sub.3 Sn layer in the composites. However, in this known Nb tube technique, since the niobium tube is utilized instead of the niobium based alloy rod, problems different from those observed in case of using the niobium based alloy rod arise during the reducing operation as the operation with high working rate progresses. That is, in case of fabricating a fine filamentary pipe in which the reducing degree of the composite from the time of initial assembling of the composite is over 10.sup.4, injuries will be formed on the wall of the niobium tube composite in the form of a filamentary pipe, hereinafter referred to as filament injury, and in addition, breakages of the niobium tube composite in the form of a filamentary pipe will occur, hereinafter referred to as filament breakage. When a superconductor filament composed of the niobium tube with these filament injuries or breakages is heat treated, tin contained in the niobium tube adversely disperses outside the tube to thereby lower the critical current (I.sub.c) or contaminate the copper matrix with the tin, which may result in instability of the cooling thereof. In case that the reducing working is carried out at the low reducing rate so that such adverse phenomena as described above will not occur, the niobium tube thus worked is obliged to have a considerably large diameter and the Nb.sub.3 Sn layer formed by the heat treatment is obliged to be considerably thick, which may result in the lowering of the critical bending strain (.epsilon..sub.bc) or increasing of the AC (alternating current) loss, thus being disadvantageous.
With this method, also, as well as the bronze method, internal diffusion method, and external diffusion method, it is difficult to increase the Sn content in the Cu-Sn composite, and with a multifilament superconductor suitable for a practical use in which each filament has a diameter of less than several tens of .mu.m, it is not practical to increase the Sn content over about 20 weight %.
In the foregoing description, the reducing rate is prescribed by (cross sectional area of a composite before the reducing working--that after the reducing working)/(cross sectional area of a composite before the reducing working).times.100 (%), and the reducing degree is represented by a ratio of (cross sectional area of a composite before the reducing working) to (that after the reducing working).